1. Field of the Invention
This invention relates to the packaging of semiconductor devices, and in particular to a ball grid array (BGA) package.
2. Description of the Related Art
BGA packages for semiconductor devices are becoming increasingly common. A cross-section of one conventional plastic BGA (PBGA) package is illustrated in FIG. 1. As shown, the package comprises a semiconductor device 10 mounted on a substrate 12 and electrically connected to traces (not shown) on the substrate 12 through bonding wires 14. Solder balls 16 serve to mount the package to a printed circuit board. The traces are connected to the solder balls 16 through vias 17 in the substrate. The substrate 12 comprises three material layers 20, 22 and 24, which comprise pre-preg, a core which is typically BT resin and pre-preg, respectively; the pre-preg thickness is approximately 00.100 mm while the core thickness is approximately 0.500 mm.
The three material layers 20, 22 and 24 define four metal layers, 26, 28, 30 and 32. Layer 26 comprises a plurality of traces for signal input/output (I/O) and layer 2820 comprises a metal plane which serves as a reference (ground) to the traces on layer 26. The traces on layer 26 may also be routed underneath the die. All of the traces on layer 26, whether or not routed underneath the die, are connected by vias to layer 32, where signal traces connect vias to the desired solder balls. Layer 30 comprises a partial ground plane and a partial power plane wherein the partial ground plane serves as a reference to the traces on layer 32. (The partial ground plane is generally directly above the traces for which it is serving as a reference while the power plane may be, for example, a ring that surrounds the ground plane and is not directly above any of those traces.)
It is desirable to improve upon various characteristics package illustrated in FIG. 1. In particular, traces on the layer 32 must be grouped together and can only be routed in areas where it is possible to provide a reference plane above on layer 30; these restrictions decrease the available amount of routing space.
These and other needs are met by the present invention, which comprises a package with multiple routing layers. In particular, according to one embodiment, the present invention comprises a substrate having a top surface with traces thereon and a bottom surface with solder balls thereon, the substrate comprising at least three material layers defining at least four substantially planar metal layers, wherein one of the metal layers comprises a reference layer that serves as a reference to both traces on a routing metal layer above the reference layer and traces on a routing metal layer below the reference layer. A semiconductor die is mounted to the substrate and bonding wires electrically connect the semiconductor die to the traces on the top surface of the substrate. The traces on the top surface of the substrate are electrically connected to the solder balls through vias and possibly through routing on another metal layer.
Since a single layer serves as a reference to two trace routing layers, an additional reference layer is not necessary, which frees up routing space.